1. Field of the Invention
The present invention relates to a pulse width modulation circuit, and more particularly to a pulse width modulation circuit capable of outputting pulses having a sharp rising edge and a sharp falling edge.
2. Description of the Related Art
FIG. 6 is a circuit diagram illustrating a conventional pulse width modulation circuit 61. FIG. 7A and FIG. 7B are waveform diagrams illustrating the output of the pulse width modulation circuit 61, wherein FIG. 7A illustrates the output from an output section a, and FIG. 7B illustrates the output from an output section b. In the pulse width modulation circuit 61, transistors TR61 and TR62 are alternately turned ON/OFF by charging/discharging condensers C61 and C62, thereby outputting pulses from the output sections a and b. An audio signal is input to the base of a transistor TR63 to control the ratio between a collector current I61 of the transistor TR63 and a collector current I62 of a transistor TR64, thereby controlling the charging time of the condensers C61 and C62 to change the output pulse width.
FIG. 8 is a circuit diagram illustrating another conventional pulse width modulation circuit 81. FIG. 9A and FIG. 9B are waveform diagrams illustrating the output of the pulse width modulation circuit 81. The operation of the pulse width modulation circuit 81 is basically the same as that of the pulse width modulation circuit 61 of FIG. 6, except that the polarities of the power supplies and the transistors are inverted.
A problem with the conventional pulse width modulation circuit 61 is the dull rising edge of the output pulses, as illustrated in FIG. 7A and FIG. 7B, thereby failing to obtain pulses that precisely correspond to the input signal. Therefore, the pulse width modulation circuit 61, when employed in an audio circuit, may deteriorate the sound quality or cause distortion. Specifically, the potential at the output section a transitions from xe2x88x92Vcc to +Vcc immediately after the transistor TR61 transitions from ON to OFF. However, since the output section a is connected to the power supply +Vcc via a resistor R61, the transition of the potential at the output section a to +Vcc is gradual, thereby resulting in a dull rising edge as illustrated in FIG. 7A. On the other hand, the potential at the output section b transitions from xe2x88x92Vcc to +Vcc immediately after the transistor TR62 transitions from ON to OFF. However, since the output section b is connected to the power supply +Vcc via a resistor R62, the transition of the potential at the output section b to +Vcc is gradual, thereby resulting in a dull rising edge as illustrated in FIG. 7B.
Similarly, a problem with the pulse width modulation circuit 81 is the dull falling edge of the output pulses, as illustrated in FIG. 9A and FIG. 9B, for the same reason as that for the pulse width modulation circuit 61.
It is an object of the present invention to provide a pulse width modulation circuit capable of outputting pulses having a sharp rising edge and a sharp falling edge.
In one embodiment of the present invention, a pulse width modulation circuit includes: pulse generation means being provided with a first power supply and charged with a first current and a second current between which a constant current is distributed to turn ON/OFF a switching element, so as to generate a pulse from a first output section; pulse modulation means for controlling the charging by controlling a distribution ratio between the first current and the second current based on an input signal, so as to change a pulse width of the output pulse according to a charging time; and first short circuit means for shorting the first output section with the first power supply when the pulse being output from the first output section transitions to a voltage of the first power supply.
Thus, the potential at the first output section can be brought to the first power supply voltage instantaneously. Therefore, the rising (or falling) edge of the pulse output from the first output section, which would otherwise be dull, can be made very sharp.
In a preferred embodiment, the pulse generation means includes a first switching element connected to the first output section, and a second switching element; and the first short circuit means includes a third switching element between the first output section and the first power supply, the first short circuit means shorting the first output section with the first power supply by turning ON the third switching element.
Thus, when the pulse being output from the first output section transitions to the first power supply voltage, the first output section and the first power supply can be shorted with each other by turning ON the third switching element.
In a preferred embodiment, the third switching element is turned ON in response to the second switching element being turned ON, and the third switching element is turned OFF in response to the second switching element being turned OFF.
Thus, when the pulse being output from the first output section transitions to the first power supply voltage, the first output section and the first power supply can reliably be shorted with each other. Moreover, since the third switching element is turned OFF in response to the second switching element being turned OFF, the first switching element and the third switching element will not be ON at the same time. Therefore, it is possible to prevent a switching element from being damaged by a through current from the first power supply.
In a preferred embodiment, the third switching element is a transistor; and a base of the third switching element is connected between the second switching element and the first power supply, an emitter thereof is connected to the first power supply, and a collector thereof is connected between the first switching element and the first power supply.
Thus, based on whether the second switching element is ON or OFF, the base-emitter voltage of the third switching element is determined, so as to turn ON or OFF the third switching element.
In a preferred embodiment, the pulse generation means further includes a second output section, wherein the pulse generation means is charged with the first current and the second current to turn ON/OFF a switching element, so as to generate a pulse from the second output section; and the pulse width modulation circuit further includes second short circuit means for shorting the second output section with the first power supply when the pulse being output from the second output section transitions to a voltage of the first power supply.
Thus, the potential at the second output section can also be brought to the first power supply voltage instantaneously. Therefore, the rising (or falling) edge of the pulse output from the second output section, which would otherwise be dull, can be made very sharp.
In a preferred embodiment, the pulse generation means includes a first switching element connected to the first output section, and a second switching element connected to the second output section; the first short circuit means includes a third switching element between the first output section and the first power supply, the first short circuit means shorting the first output section with the first power supply by turning ON the third switching element; and the second short circuit means includes a fourth switching element between the second output section and the first power supply, the second short circuit means shorting the second output section with the first power supply by turning ON the fourth switching element.
Thus, when the pulse being output from the second output section transitions to the first power supply voltage, the second output section and the first power supply can be shorted with each other by turning ON the fourth switching element.
In a preferred embodiment, the third switching element is turned ON in response to the second switching element being turned ON, and the third switching element is turned OFF in response to the second switching element being turned OFF; and the fourth switching element is turned ON in response to the first switching element being turned ON, and the fourth switching element is turned OFF in response to the first switching element being turned OFF.
Thus, since the fourth switching element is turned ON in response to the first switching element being turned ON, the second output section and the first power supply can reliably be shorted with each other when the pulse being output from the second output section transitions to the first power supply voltage. Moreover, since the fourth switching element is turned OFF in response to the first switching element being turned OFF, the second switching element and the fourth switching element will not be ON at the same time. Therefore, it is possible to prevent a switching element from being damaged by a through current from the first power supply.
In a preferred embodiment, the third switching element is a transistor; a base of the third switching element is connected between the second switching element and the first power supply, an emitter thereof is connected to the first power supply, and a collector thereof is connected between the first switching element and the first power supply; the fourth switching element is a transistor; and a base of the fourth switching element is connected between the first switching element and the first power supply, an emitter thereof is connected to the first power supply, and a collector thereof is connected between the second switching element and the first power supply.
Thus, based on whether the first switching element is ON or OFF, the base-emitter voltage of the fourth switching element is determined, so as to turn ON or OFF the fourth switching element.
In a preferred embodiment, the pulse generation means includes an astable multivibrator.
In a preferred embodiment, the pulse generation means is provided further with a second power supply; and the voltage of the first power supply is set to be higher than that of the second power supply.
Thus, the rising edge of the pulse output from the first output section can be made very sharp.
In a preferred embodiment, the pulse generation means is provided further with a second power supply; and the voltage of the first power supply is set to be lower than that of the second power supply.
Thus, the falling edge of the pulse output from the first output section can be made very sharp.
In a preferred embodiment, the pulse generation means is provided further with a second power supply, and includes a first transistor, a second transistor, a first condenser and a second condenser; a collector of the first transistor is connected between one end of the first condenser and the first power supply, an emitter thereof is connected to the second power supply, and a base thereof is connected to one end of the second condenser; and a collector of the second transistor is connected between the other end of the second condenser and the first power supply, an emitter thereof is connected to the second power supply, and a base thereof is connected to the other end of the first condenser.
In a preferred embodiment, the pulse modulation means includes: a constant current circuit for generating a constant current; a fifth transistor for receiving the input signal at its base to control the first current; and a sixth transistor for controlling the second current based on the first current.